The present invention relates to an image forming apparatus, such as an electrophotographic copying machine or a laser printer, and more particularly to an image forming apparatus incorporating an image carrier (for example, a photosensitive drum) around which a toner image will be formed and which is rotated around a predetermined axis; an endless belt serving as an intermediate transfer member so as to form an image; the intermediate transfer belt arranged to be moved in a predetermined movement direction along a predetermined loop passage while the transfer belt is making contact with the outer surface of the image carrier; and a transfer roller disposed opposite to the image carrier across the transfer belt in such a manner as to press the transfer belt against the image carrier and arranged to be rotated around a predetermined axis so as to transfer the toner image formed on the image carrier to the transfer belt.
An image forming apparatus, such as a copying machine or a printer of an electrophotographic type, is adapted to a method by which a non-fixed toner image formed on a latent-image carrier, such as a photosensitive drum, is transferred to a recording medium, such as a paper sheet so as to print an image. As a method of transferring the non-fixed toner image to the recording medium, a method is known whereby the non-fixed toner image is directly transferred to the recording medium. Another method is known in which the non-fixed toner image is primarily transferred to the surface of an intermediate transfer member in the form of a drum or an endless-belt film member. Then, the non-fixed toner image on the intermediate transfer member is secondarily transferred to a recording medium so as to obtain a copied image.
FIG. 17 is a schematic view showing an essential structure of a color printer taken as an example of the image forming apparatus incorporating the belt-type intermediate transfer member. Referring to the drawing, the surface of a latent image carrier, such as a photosensitive drum, (hereinafter represented by a "photosensitive drum") 1, is uniformly electrically charged by an electric charger 2 with a predetermined electric charge. An image writing unit (ROS) (not shown) performs writing scan using a laser beam L so that an electrostatic latent image corresponding to a first-color image signal is formed. The electrostatic latent image reaches a position opposite to a first-color developing unit of the developing unit 3 when the photosensitive drum 1 has been rotated in a direction A. Then, the electrostatic latent image is developed by the first-color developing unit using toner. The photosensitive drum 1 furthermore rotates while carrying a toner image T.
In synchronization with the toner developing operation, the intermediate transfer belt 4 is moved at substantially the same velocity as the peripheral velocity of the photosensitive drum 1. In a primary transferring portion, a primary transfer roller 5 is disposed adjacent to a position immediately below a position at which the photosensitive drum 1 and the intermediate transfer belt 4 are in contact with each other. The primary transfer roller 5 is disposed to be in contact with the intermediate transfer belt 4. In the primary transferring portion, the toner image T carried by the photosensitive drum 1 is primarily transferred to the intermediate transfer belt 4 with a transferring electric field (a transferring bias) having a polarity opposite to the polarity charged to the toner which is applied to the primary transfer roller 5. Thus, a primary transferring rotation for one color is completed.
As a result of a rotational movement of the intermediate transfer belt 4, the toner image primarily transferred to the intermediate transfer belt 4 arrives at a secondary transferring portion in which a secondary transfer roller 6 is disposed. In a case of a full color image forming apparatus, the process from formation of the latent image to the primary transference of the toner image is repeated for predetermined colors (in general, yellow (Y), magenta (M), cyan (C) and black (Bk)). Thus, a color toner image is formed on the intermediate transfer belt 4 by superimposing the toner in a plurality of colors.
To form a toner image in each color, the developing unit 3 comprises a rotary unit consisting of developing units in four color, that is, a yellow developing unit 3.sub.-1, a magenta developing unit 3.sub.-2, a cyan developing unit 3.sub.-3 and a black developing unit 3.sub.-4. Thus, latent images in the foregoing colors formed on the photosensitive drum 1 can sequentially be developed.
A toner image in a first color carried by the photosensitive drum 1 is transferred to the intermediate transfer belt 4 in the primary transferring portion. Then, a residual portion of the toner on the photosensitive drum 1 is removed by a photosensitive-member cleaner 7. Moreover, a destaticizer (not shown) neutralizes the electric charge. Then, a latent image corresponding to a second color is formed. The latent image in the second color is developed similarly to that in the first color. The toner image in the second color is superimposed on the transferred toner image in the first color and formed on the intermediate transfer belt 4 so as to be transferred. Toner images in the third and following colors are similarly transferred to the intermediate transfer belt 4 in a superimposition manner. As a result, a color toner image is formed on the intermediate transfer belt 4 by superimposing non-fixed toner in the plural colors.
When the intermediate transfer belt 4, to which the toner images in all of the colors have primarily been transferred, reaches a secondary transferring position, a recording paper sheet P which is a recording medium fed from the paper feeding tray 8 at adjusted timing is moved to the secondary transferring position.
When the recording paper sheet P is held by the secondary transfer roller 6 and the intermediate transfer belt 4 so as to be moved, the toner image on the intermediate transfer belt 4 is secondarily transferred to the recording paper sheet P with a transferring electric field produced by transferring voltage having a polarity opposite to the polarity charged to the toner image. The recording paper sheet P to which the toner image has secondarily be transferred is moved to the fixing unit 9 so as to be subjected heating and pressurizing processes so as to fix the toner image to the recording paper sheet P. Thus, an image forming process is completed.
The secondary transfer roller 6 is disposed to be brought into contact with the intermediate transfer belt 4 and separated from the same in a direction indicated with an arrow C. In synchronization with introduction of the recording paper sheet P, the secondary transfer roller 6 makes contact with the intermediate transfer belt 4. In synchronization with discharge, the secondary transfer roller 6 is separated from the intermediate transfer belt 4. When the secondary transference has been completed, the secondary transfer roller 6 is returned to a standby position. Also a cleaner 10 disposed opposite to the intermediate transfer belt 4 is brought into contact with the intermediate transfer belt 4 and separated from the same similarly to the second transfer roller 6 to clean the toner image omitted from transference to the recording paper sheet P.
The intermediate transfer belt 4 is arranged by a drive roller 11, an idle roller 12, a secondary-transference backup roller 13 and a tension roller 14 so as to be moved in a direction indicated with an arrow B by the drive roller 11. The intermediate transfer belt 4 is provided with ribs (not shown) so that the positions in the axial directions of each roller including the drive roller 11 are limited, the ribs being provided on the inner periphery of the side portions. To reinforce the intermediate transfer belt, a belt reinforcing tape is applied to the outer periphery of the side portions.
The color image forming apparatus incorporating the intermediate transfer belt is arranged to collectively transfer, to the recording medium, a synthesized toner image (an image formed by superimposing color toner images) which has been transferred to the intermediate belt in the superimposition manner. Therefore, the foregoing apparatus has an advantage that deviation of the positions of toner images and disorder of a formed image can effectively be prevented when a method is employed in which color toner images are sequentially and directly transferred from the latent image carrier to the recording medium. Note that an example of the image forming apparatus incorporating the intermediate transfer belt is disclosed in Unexamined Japanese Patent Publication No. 5-134556.
However, the image forming apparatus incorporating the intermediate transfer belt has the following problems. An ideal structure of the above-mentioned image forming apparatus is that the peripheral velocity VD of the photosensitive drum 1 and the movement velocity VB of the intermediate transfer belt 4 are the same, that is, the relative velocity is zero. However, the outer diameter of the drive roller 11 of the intermediate transfer belt 4 and that of the photosensitive drum 1 have tolerances. Therefore, the two velocities are slightly different from each other and thus a relative velocity is inevitably produced.
When a transferring bias is applied to the primary transfer roller 5, the photosensitive drum 1 and the intermediate transfer belt 4 attract each other so as to be brought into intimate contact with each other. Since the photosensitive drum 1 and the intermediate transfer belt 4 brought into intimate contact with each other have a relative velocity, forces are exerted on the photosensitive drum 1 and the intermediate transfer belt 4.
An assumption is made that the velocity VD of the photosensitive drum 1 is larger than the velocity VB of the intermediate transfer belt 4. Force in the rotating direction B of the intermediate transfer belt 4 is exerted on the intermediate transfer belt 4. The force causes the intermediate transfer belt 4 to be loosened between the primary transfer roller 5 and the drive roller 11. On the other hand, force opposing the rotational direction A of the photosensitive drum 1 is exerted on the photosensitive drum 1. As a result, the position of the photosensitive drum 1 is shifted. The looseness of the intermediate transfer belt 4 and shift of the position of the photosensitive drum 1 cause color deviation when multi-color toner is transferred.
The forces exerted on the intermediate transfer belt 4 and the photosensitive drum 1 are produced when the foregoing elements are brought into intimate contact with each other. Therefore, if their friction coefficients are small, color deviation satisfies an allowable range. If toner particles exist on the photosensitive drum 1, the toner particles serve as a material for reducing friction. Thus, the frictional force can be reduced.
After the toner images on the intermediate transfer belt 4 have been transferred to the recording paper sheet P, the intermediate transfer belt 4 is cleaned by the cleaner 10 so that the toner particles are removed. Therefore, large frictional force of the photosensitive drum 1 and the intermediate transfer belt 4 is exerted immediately before the primary transference of the first color of a first print. However, the frictional force is reduced after start of the primary transference to transfer the toner image to the intermediate transfer belt 4. Also in the transference of the second and following colors which is performed after the process for the first color has been performed and in the all color transference of two and following pages of a continuous job, existence of toner particles results in reduction in the frictional force.
As a result, the extent of the deviation of the photosensitive drum 1 and that of the looseness of the intermediate transfer belt 4 vary between start of the primary transference of the first color and a following moment of time. As a result of the foregoing phenomenon, there arises a problem of the intermediate transfer belt 4 in that the position of the leading end of an image in only the first color image of a first page is deviated from second and following colors. The foregoing problem can be prevented when combination of the materials of the photosensitive drum 1 and the intermediate transfer belt 4 has a small coefficient of friction. However, a combination of materials of the photosensitive drum 1 and the intermediate transfer belt 4 having characteristics required to form an image and enabling the frictional force to be reduced has not been known.
Besides, in Unexamined Japanese Patent Publication No. 9-152791, there is disclosed an image forming apparatus in which toner images in all colors are transferred to the intermediate transfer belt by a transfer roller and a final toner image is thus formed thereon, and the final toner image is then transferred therefrom to a paper sheet to form a color image.
The color image forming apparatus incorporating the intermediate transfer belt is structured to collectively transfer, to a paper sheet, a synthesized toner image (an image obtained by superimposing color toner images) formed on the intermediate transfer belt in the superimposition manner. Therefore, the foregoing apparatus has an advantage that displacement of toner images and disorder of the formed image experienced with the method in which color toner images are sequentially and directly transferred from the photosensitive drum to a paper sheet can effectively be prevented.
The above-mentioned color image forming apparatus incorporates the transfer roller for transferring toner images from the photosensitive drum to the intermediate transfer belt. The transfer roller is disposed opposite to the photosensitive drum across the intermediate transfer belt and urged in such a manner as to press the intermediate transfer belt against the photosensitive drum. At this time, the transfer roller is usually urged at positions adjacent to two axial directional ends.
Therefore, the pressing load generated by the transfer roller is enlarged at each of both end portions as compared with that at a central portion with respect to axial direction of the roller. Therefore, the width (hereinafter called a "nip width") of the contact region between the intermediate transfer belt and the photosensitive drum in a conveying direction of the belt is enlarged at the both end portions as compared with the central portion. Namely, in the contact region T between the intermediate transfer belt and the photosensitive drum shown in FIG. 14, nip width at the central portion W1 and nip width at the both end portions W2 satisfy the following relationship:
W1&lt;W2.
If the nip width values are dispersed in the axial direction, a synthesized resistance value including the photosensitive drum, the transfer roller and the intermediate transfer belt is dispersed in the axial direction. When toner images formed on the image carrier are transferred to the intermediate transfer belt by the transfer roll, a uniform transferring electric field cannot be realized. As a result, there is apprehension that partial irregularity in transference occurs. Therefore, the conventional apparatuses must raise the transferring voltage so as to intensify the transferring electric field in order to prevent partial irregularity occurring in a transferring operation.
If the transferring voltage is raised, the transferring voltage, however, is raised excessively in a portion in which the synthesized resistance value is small. As a result, a transferring current is introduced into toner on the intermediate transfer belt. Therefore, the charged polarity of a portion of toner is inverted, causing a defect to take place in transference. As described above, the color image forming apparatus incorporating the intermediate transfer belt cannot easily realize excellent transferring performance by optimizing the transferring voltage.